Unrecovered NSGenericException while parsing CSS file

    Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSArrayM: 0x5db8f00> was mutated while being enumerated.(
    "font-size",
    color,
    "font-size"
    )'

Apparently there is a case where the '__kRuleSetOrder__' array under 'existingProperties' and '_mutatingRuleset' will be the same. Couldn't reproduce it with a test but have managed to identify a potential minimum .css that triggers. There are however other preconditions that must exist. Since the preconditions can't be pinpointed, can't be sure if they are part of a valid NimbusCSSParser state.